BWRs have conventionally utilized active safety systems to control and mitigate accident events. Those events varied from small break to design basis accidents. Passive safety systems have been studied for use in simplified BWRs (SBWRs) because of their merits in reducing specialized maintenance and surveillance testing of the safety-related equipment, and in eliminating the need for AC power, thereby improving the reliability of essential safety system responses necessary for the control and mitigation of adverse effects produced by accidents. SBWRs can additionally be designed with certain passive safety features that provide more resistance to human error in accident control and mitigation.
Referring to FIG. 1, an SBWR includes a reactor pressure vessel 10 containing a nuclear reactor fuel core 12 submerged in water 14. The fuel core heats the water to generate steam 14a which is discharged from the reactor pressure vessel through a main steam line 16 and used to power a steam turbine-generator for producing electrical power.
The reactor pressure vessel is surrounded by a containment vessel 18. The volume inside containment vessel 18 and outside reactor pressure vessel 10 is called the drywell 20. The containment vessel is a concrete structure having a steel liner and is designed to withstand elevated pressure inside the drywell. The drywell typically contains a noncondensable gas such as nitrogen.
In accordance with the conventional SBWR containment design, an annular suppression or wetwell pool 22 surrounds the reactor pressure vessel within the containment vessel. The suppression pool is partially filled with water 24 to define a wetwell airspace or plenum 26 thereabove. The suppression pool 22 serves various functions including being a heat sink in the event of certain accidents. For example, one type of accident designed for is a loss-of-coolant accident (LOCA) in which steam from the reactor pressure vessel 10 leaks into the drywell 20. Following the LOCA, the reactor is shut down but pressurized steam and residual decay heat continue to be generated for a certain time following the shutdown. Steam escaping into the drywell 20 is channeled into the suppression pool 22 through a multiplicity of (e.g., eight) vertical flow channels, each flow channel 27 having plurality of (e.g., three) horizontal vents 28. Steam channeled into the suppression pool 22 through the vents 28 carries with it portions of the drywell noncondensable gas 30. The steam is condensed and the noncondensable gas 30 is buoyed upwardly to the wetwell plenum 26, where it accumulates.
When the pressure in wetwell plenum 26 exceeds that in drywell 20, one or more vacuum breakers 36, which penetrate the wetwell wall, are opened to allow noncondensable gas 30 in the wetwell plenum 26 to vent to the drywell 20. The vacuum breakers 36 remain closed when the pressure in drywell 20 is equal to or greater than the pressure in the wetwell plenum 26.
The system further includes one or more gravity-driven cooling system (GDCS) pools 38 located above the suppression pool 22 within the containment vessel 18. The GDCS pool 38 is partially filled with water 42 to define a GDCS plenum 44 thereabove. The GDCS pool 38 is connected to an outlet line 46 having a valve 48 which is controlled by controller 40. The valve 48 is opened to allow GDCS water 42 to drain by gravity into pressure vessel 10 for cooling the core following a LOCA. Steam and noncondensable gas can be channeled directly into the GDCS plenum 44 from the drywell 20 via an inlet 50. An optional condenser or heat exchanger 72 may be provided for condensing steam channeled through inlet 50 following draining of the GDCS water 42 for drawing in additional steam and noncondensable gas.
The suppression pool 22 is disposed at an elevation which is above the core 12 and is connected to an outlet line 32 having a valve 34 which is controlled by a controller 40. The valve 34 is opened after an appropriate time delay from the opening of valve 48 to allow wetwell water 24 to also drain by gravity into the pressure vessel 10 for cooling the core following a LOCA.
In the SBWR design, a passive containment cooling system (PCCS) is provided for removing heat from the containment vessel 18 during a LOCA. A condenser pool 52, configured as a collection of subpools (not shown) interconnected so as to act as a single common large pool, is disposed above the containment vessel 18 and above the GDCS pool 38. The condenser pool 52 contains a plurality of PCC heat exchangers 54 (only one of which is shown in FIG. 1), also commonly referred to as PCC condensers, submerged in isolation water 56. The condenser pool 52 includes one or more vents 58 to atmosphere outside the containment for venting the airspace above the condenser pool water 56 for discharging heat therefrom upon use of the PCC heat exchanger 54.
The PCC heat exchanger 54 has an inlet line 60 in flow communication with the drywell 20 and an outlet line 62 joined to a collector chamber 64 from which a vent pipe 66 extends into the suppression pool 22 and a condensate return conduit 68 extends into the GDCS pool 38. The PCC heat exchanger 54 provides passive heat removal from the drywell 20 following the LOCA, with steam released into the drywell flowing through inlet 60 into the PCC heat exchanger wherein it is condensed. The noncondensable gas (e.g., nitrogen) within the drywell is carried by the steam into the PCC heat exchanger and must be separated from the steam to provide effective operation of the PCC heat exchanger. The collector chamber 64 separates the noncondensable gas from the condensate, with the separated noncondensable gas being vented into the suppression pool 22, and the condensate being channeled into the GDCS pool 38. A water trap or loop seal 70 is provided at the end of condensate return conduit 68 in GDCS pool 38 to restrict backflow of heated fluids from GDCS pool 38 to suppression pool 22 via the condensate return conduit 68, which would bypass PCC heat exchanger 54.
Accordingly, this system is configured to transport the noncondensable gas from the drywell 20 to the wetwell plenum 26 and then condense steam from the drywell in the PCC heat exchanger 54. The noncondensable gas will remain in the enclosed wetwell until the PCC heat exchanger 54 condenses steam faster than it is released from the reactor pressure vessel. When this occurs, the PCC heat exchanger lowers the drywell pressure below that of the wetwell, which causes the vacuum breakers 36 to open, thereby allowing noncondensable gas stored in the wetwell to return to the drywell.
Furthermore, the noncondensable gas within the drywell is also carried directly into the wetwell by the escaping steam which is channeled through horizontal vents 28. The steam is released underwater in the wetwell and condenses therein, while the noncondensable gas is buoyed upwardly through the pool water to vent into the enclosed wetwell air chamber disposed above the pool water, where the noncondensable gas is retained. As the noncondensable gas accumulates in the wetwell chamber, the pressure therein increases correspondingly. Accordingly, the overall containment pressure remains relatively high after the LOCA due to the high-pressure steam escaping into the drywell, and the wetwell pressure also remains high due to the accumulation of noncondensable gas. The consequences of post-LOCA elevated pressure levels obviously lead to requirements for robustness of the containment structures which must be provided to resist and contain these pressures, and in turn lead to high nuclear plant capital costs through extensive and sophisticated, costly, structural design and construction practices, premium-priced construction materials, and still other consequences which translate into elevated costs for nuclear power plants.